Conventional passive infrared (PIR) sensors are predominantly used in movement detectors, but although they are very inexpensive, they do not provide any spatial resolution and have difficulty detecting objects having low temperature contrasts as compared to their surroundings. Doppler detectors or movement detectors using the PIR principle and the Doppler principle also do not provide any spatial resolution. It is precisely this property, however, which is required not only for determining whether an object is located in a room under surveillance, but also for determining where the object is located in the room, in which direction it is moving, and the type or class of object concerned.
An obvious use of so-called "thermal-image sensors," i.e., image-providing sensors operating in the wavelength region of about 5 to 15 .mu.m, is undesirable in that conventional thermal-image sensors are so expensive that sufficiently high-resolution sensors cannot be used for movement detectors. As such, high resolution applications using thermal-image sensors are not practical.
Additionally, images of objects taken with conventional low-resolution thermal-image sensors, having in the range of about 4.times.4 pixels up to 32.times.32 pixels, often cannot be analyzed precisely enough for the required application. For example, such a resolution would be too low for distinguishing humans from non-human animals. Also, conventional thermal-image sensors have a low detection sensitivity for low temperature contrast at ambient temperatures around 30.degree. C.
So-called "image sensors" are also known, which are image-providing sensors operating in the visible and near-infrared range, particularly in the wavelength range from about 0.4 to 1.8 .mu.m. Conventional image sensors are inexpensive and widely used, but are generally used in environments having a minimal level of brightness. These sensors suffer the shortcoming that they are unable to function properly in the dark unless combined with a lighting system. In addition, to evaluate the signal of a conventional image sensor, the entire image always has to be processed, which requires a relatively high expenditure of memory capacity and computer processing time and, if the evaluation is not carried out locally, requires an expensive transmission of data across a communications media.
If low-resolution image sensors or those having the possibility of reading-out images with reduced resolution are used there is the risk that low contrast objects may be blurred and can therefore no longer be detected.